Question: Which of these numbers is prime? ${13,\ 25,\ 55,\ 87,\ 98}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 13 are 1 and 13. The factors of 25 are 1, 5, and 25. The factors of 55 are 1, 5, 11, and 55. The factors of 87 are 1, 3, 29, and 87. The factors of 98 are 1, 2, 7, 14, 49, and 98. Thus, 13 is a prime number.